Two section blue laser diode with reduced output power droop

ABSTRACT

A III-V nitride blue laser diode has an amplifier region and a modulator region. The amplifier region has a constant current to keep the region near the lasing threshold. The modulator region has a small varying forward current or reverse bias voltage which controls the light output of the laser. This two section blue laser diode requires much lower power consumption than directly modulated lasers which reduces transient heating and “drooping” of the light output.

BACKGROUND OF THE INVENTION

The present invention relates to a blue laser diode and, moreparticularly, to a two section blue laser diode with an amplifier regionand a modulator region to reduce power output variations.

Solid state lasers, also referred to as semiconductor lasers or laserdiodes, are well known in the art. These devices generally consist of aplanar multi-layered semiconductor structure having one or more activesemiconductor layers bounded at their ends by cleaved surfaces that actas mirrors. The semiconductor layers on one side of the active layer inthe structure are doped with impurities so as to have an excess ofmobile electrons. The semiconductor layers on the other side of theactive layer in the structure are doped with impurities so as to have adeficiency of mobile electrons, therefore creating an excess ofpositively charged carriers called holes. Layers with excess electronsare said to be n-type, i.e. negative, while layers with excess holes aresaid to be p-type, i.e. positive.

An electrical potential is applied through electrodes between the p-sideand the n-side of the layered structure, thereby driving either holes orelectrons or both in a direction perpendicular to the planar layersacross the p-n junction so as to “inject” them into the active layers,where electrons recombine with holes to produce light. Optical feedbackprovided by the cleaved mirrors allows resonance of some of the emittedlight to produce coherent “lasing” through the one mirrored edge of thesemiconductor laser structure.

Semiconductor laser structures comprising group III-V nitridesemiconductor layers grown on a sapphire substrate will emit light inthe near ultra-violet to visible spectrum within a range including 360nm to 650 nm.

The shorter wavelength of blue/violet laser diodes provides a smallerspot size and a better depth of focus than the longer wavelength of redand infrared (IR) laser diodes for laser printing operations and highdensity-optical storage. In addition, blue lasers can potentially becombined with existing red and green lasers to create projectiondisplays and color film printers.

The III-V nitrides make possible diode lasers that operate at roomtemperature and emit shorter-wavelength visible light in the blue-violetrange under continuous operation. The III-V nitrides comprise compoundsformed from group III and V elements of the periodic table. The III-Vnitrides can be binary compounds such as gallium nitride (GaN), aluminumnitride (AlN), or indium nitride (lnN), as well as ternary alloys ofaluminum gallium nitride (AlGaN) or indium gallium nitride (InGaN), andquartemary alloys such as aluminum gallium indium nitride (AlGaInN).

These materials are highly promising for use in short-wavelength lightemitting devices for several important reasons. Specifically, theAlGaInN system has a large bandgap covering the entire visible spectrum.III-V nitrides also provide the important advantage of having a strongchemical bond which makes these materials highly stable and resistant todegradation under high electric current and intense light illuminationconditions that are present at active regions of the devices. Thesematerials are also resistant to dislocation formation once grown.

High speed and high resolution printing requires laser devices withlittle or no fluctuations of the output power. For example, thevariation in the laser light output required for red and IR laser diodesfor printing applications is smaller than 4% and those requirementswould be similar for AlGaInN laser diodes.

Heat is generated through voltages drops across the metalelectrode/semiconductor interfaces, which have a finite resistance, andthrough voltage drops across the resistive semiconductor layers. Energyis also introduced into the active region of the laser by injectingelectrons into the conduction band and/or holes into the valence band.Electrons relax into the lowest energy state of the conduction band andholes relax into the lowest energy state of the valence band throughnon-light emitting processes and release their energy in the form ofheat.

When a laser device is switched from the OFF to the ON state, transientheating, or heating that changes over time, can cause the light outputof AlGaInN laser diodes to drop significantly.

As an illustrative example, an AlGaInN blue laser diode is forwardbiased with a constant current above the lasing threshold current. Atthe initial time t=0, with a constant current of 65 mA, the blue laserdiode will have a first output power PI of 9.5 mW with a laser structuretemperature of 20 degrees C., as shown in FIG. 1.

However, as time increases with the blue laser diode above lasingthreshold with the constant current, the temperature of the laserstructure increases. This increased temperature results in a decreasedoutput power for the AlGaInN laser diode.

At a subsequent time t=∞, still with a constant current of 65 mA, theblue laser will have a second output power P2 of 6.2 mW with a laserstructure temperature of 30 degrees C., as shown in FIG. 1. The secondoutput power P2 is lower than the initial output power P1. Thus, theplot of output power versus time of FIG. 2 shows an initial output powerof P1 at turn-on, “drooping” to the second lower output power P2 as theblue laser diode is operated.

Thermal fluctuations are especially deleterious to maintaining constantoptical power output, especially during pulsed modulation. In virtuallyall of the applications of these lasers, it is necessary to modulate theoutput of the laser into a series of pulses.

Transient heating during a sequence of pulses can have a cumulativeeffect on the temperature depending on the number and frequency of thepulses. For example, if the time between successive pulses is large, thelaser diode will be given sufficient time to cool, so that theapplication of the driving current has a large temperature effect (i.e.,a large droop in output power will occur at turn-on of the next pulse).The shorter the time between pulses, the less time the laser diode hasto cool between one pulse and the next, leading to a sustained increasein the temperature of the laser. This sustained temperature increaseresults in a further decrease in the output pulse obtained with aconstant level of input current.

Another related consequence of transient heating of a laser iswavelength variation during a pulse and over long streams of pulses.Essentially, the operating wavelength of a laser diode is dependent onthe temperature of the laser diode. If the temperature varies, thewavelength of operation will vary. The effect of this variation ofwavelength, for example in the laser xerography application, is to varythe energy that can be written onto the photoreceptor. This can alsotranslate directly into variations in the spot size and pattern on thephotoreceptor.

Digital printing requires accurate control of the optical energydelivered in each pulse. In systems currently known to those skilled inthe art, a predetermined amount of energy is delivered in each pulse byturning on the optical beam to a desired power level for a fixed timeinterval. This approach requires that the laser output power bereproducible from pulse to pulse and constant during a pulse, in orderthat the optical energy delivered in each pulse be accuratelycontrolled. Accurate control is especially important in printing withdifferent grey levels formed by varying the number of exposed spots orwhen exposing very closely spaced spots in order to control theformation of an edge.

Due to the poor thermal conductivity of the sapphire substrate and therelatively high electric power consumption of III-nitride baser laserdevices, transient heating is an issue to AlGaInN devices. For example,AlGaInN laser devices have threshold currents in the order of 50 mA andoperating voltages of 5 V (compared to about 15 mA and 2.5 V for redlasers).

It is an object of the present invention to provide a blue laser withreduced power output variations due to transient heating.

SUMMARY OF THE INVENTION

According to the present invention, an III-V nitride blue laser diodehas an amplifier region and a modulator region. The amplifier region hasa constant current to keep the region near the lasing threshold. Themodulator region has a small varying forward current or reverse biasvoltage which controls the light output of the laser. This two sectionblue laser diode requires much lower power consumption than directlymodulated lasers which reduces transient heating and “drooping” of thelight output.

The light output of the laser diode is controlled by absorption changesin the modulator region. Absorption changes can be induced using fieldeffects (e.g. QCSE) or carrier effects (e.g. band filling or fieldscreening) which require much lower power consumption than directlymodulated lasers.

Since only a small section of the laser diode is used to control theoutput power, the resulting lower capacitance should also be beneficialfor achieving higher modulation speeds.

Other objects and attainments together with a fuller understanding ofthe invention will become apparent and appreciated by referring to thefollowing description and claims taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the invention and many of the attendantadvantages thereof will be readily obtained and understood by referringto the following detailed description and the accompanying drawings inwhich like reference numerals denote like elements as between thevarious drawings. The drawings, briefly described below, are not toscale.

FIG. 1 is a plot of power output versus input current for a blue laserdiode showing the effect of transient heating of the laser diode.

FIG. 2 is a plot of power output versus time for the blue laser diode ofFIG. 1 showing power output “droop”.

FIG. 3 is a cross-sectional side view of the blue laser diode of thepresent invention after III-V nitride film growth.

FIG. 4a is a cross-sectional front view of the two sectionridge-waveguide blue laser diode with gain region and modulator regionof the present invention.

FIG. 4b is a cross-sectional side view of the two section blue laserdiode with gain region and modulator region of the present inventionseparated by an etched trench.

FIG. 4c is a cross-sectional side view of the two section blue laserdiode with gain region and modulator region of the present inventionseparated by an electrically insulating ion implanted region.

FIG. 5 is a top view of the two-section blue laser diode of FIG. 4.

FIG. 6a shows the measured light output vis. current characteristic fora blue two-section laser diode for different modulator section reversebias voltages.

FIG. 6b shows the measured light output vs. reverse bias voltagecharacteristic for a blue two-section laser diode at a constant gainsection current.

FIG. 7a shows the measured two-section laser diode emission spectra at amodulator section voltage of 0 V (OFF state).

FIG. 7b shows the measured two-section laser diode emission spectra at amodulator section voltage of 6 V (ON state).

DESCRIPTION OF THE INVENTION

In the following detailed description, numeric ranges are provided forvarious aspects of the embodiments described. These recited ranges areto be treated as examples only, and are not intended to limit the scopeof the claims hereof. In addition, a number of materials are identifiedas suitable for various facets of the embodiments. These recitedmaterials are to be treated as exemplary, and are not intended to limitthe scope of the claims hereof.

Reference is now made to FIG. 3 wherein is described the basic twosection III-V nitride based semiconductor alloy diode laser 100 of thepresent invention. The semiconductor laser structure 100 has a C-face(0001) or A-face (1120) oriented sapphire (Al₂O₃) substrate 102 on whicha succession of semiconductor layers is epitaxially deposited. The laserstructure 100 includes a thin buffer layer 103, also known as anucleation layer, formed on the sapphire substrate 102. The buffer layer103 acts primarily as a wetting layer, to provide smooth, uniformcoverage of the top surface of the sapphire substrate 102. The bufferlayer 103 can comprise any suitable material. Typically, the bufferlayer 103 is formed of a binary or ternary III-V nitride material, suchas, for example, GaN, AIN, InGaN or AlGaN. The buffer layer 103typically has a thickness of from about 10 nm to about 30 nm. The bufferlayer 103 is typically undoped.

A second III-V nitride layer 104 is formed on the buffer layer 103. Thesecond III-V nitride layer 104 is an n-type GaN or AlGaN layer. Thesecond III-V nitride layer 104 acts as a lateral n-contact and currentspreading layer. The second III-V nitride layer 104 typically has athickness of from about 1 μm to about 10 μm. The second III-V nitridelayer 104 is typically n-type GaN:Si or AlGaN:Si.

A third III-V nitride layer 105 is formed over the second III-V nitridelayer 104. The third III-V nitride layer 105 is a defect reducing layer.The third III-V nitride layer 105 typically has a thickness of fromabout 25 nm to about 200 nm. The third III-V nitride layer 105 istypically n-type InGaN:Si with an In content smaller than the InGaNquantum well(s) in the active region 108.

A fourth III-V nitride layer 106 is formed over the third III-V nitridelayer 105. The fourth III-V nitride layer 106 is an n-type claddinglayer. The fourth III-V nitride layer 106 typically has a thickness offrom about 0.2 μm to about 2 μm. The fourth III-V nitride layer 106 istypically n-type AlGaN:Si with an Al content larger than the third orthe second III-V nitride layer.

A fifth III-V nitride layer 107, which is a waveguide layer, is formedover the fourth III-V nitride layer 106. The fifth III-V nitride layer107 is typically n-type In GaN:Si, GaN:Si, InGaN:un or GaN:un with an Incontent smaller than the InGaN quantum well(s) in the active region 108.The overall thickness of the fifth III-V nitride layer 107 is typicallyfrom about 0.05 μm to about 0.2 μm.

On top of the fifth III-V nitride layer 107, the InGaN quantum wellactive region 108 is formed, comprised of at least one InGaN quantumwell. For multiple-quantum well active regions, the individual quantumwells typically have a thickness of from about 10 Å to about 100 A andare separated by InGaN or GaN barrier layers which have typically athickness of from about 10 Å to about 200 Å The InGaN quantum wells andthe InGaN or GaN barrier layers are typically undoped or can beSi-doped.

A sixth III-V nitride layer 109, which is a carrier confinement layer,is formed over the InGaN (multiple) quantum well active region 108. Thesixth III-V nitride layer 109 has a higher band gap than the quantumwell active region. The sixth III-V nitride layer 109 is typicallyp-type Al_(x)Ga_(1-x)N:Mg with an Al content in the range from x=0.05 tox=0.4. The overall thickness of the sixth III-V nitride layer 109 istypically from about 5 nm to about 100 nm.

A seventh III-V nitride layer 110, which is a waveguide layer, is formedover the sixth III-V nitride layer 109. The seventh III-V nitride layer110 is typically p-type InGaN:Mg or GaN:Mg with an In content smallerthan the InGaN multi-quantum well(s) in the active region. The overallthickness of the seventh III-V nitride layer 110 is typically from about50 nm to about 200 nm.

A eighth III-V nitride layer 111 is formed over the seventh III-Vnitride layer 110. The eighth III-V nitride layer 111 serves as a p-typecladding layer. The eighth III-V nitride layer 111 typically has athickness of from about 0.2 μm to about 1 μm. The eighth III-V nitridelayer 111 is typically AlGaN:Mg with an Al content larger than theseventh III-V nitride layer.

A ninth III-V nitride layer 112 is formed over the eighth III-V nitridelayer 111. The ninth III-V nitride layer 112 forms a p-contact layer forthe minimum-resistance metal electrode, to contact the p-side of theheterostructure 100. The ninth III-V nitride layer 112 typically has athickness of from about 10 nm to 200 nm.

The laser structure 100 can be fabricated by a technique such asmetalorganic chemical vapor deposition (MOCVD) or molecular beam epitaxyas is well known in the art. MOCVD growth is typically performed on2-inch or 3-inch diameter sapphire substrate wafer. The substrate 102can be a C-face (0001) or A-face (1120) oriented sapphire (Al₂O₃)substrate. The sapphire substrate wafers are of standard specificationsincluding an epitaxial polish on one side and a typical thickness of10-mil to 17-mil. Other examples of substrates include, but are notlimited to 4H—SiC, 6H—SiC, AlN or GaN. In case of growth on a GaNsubstrate, the second III-V nitride layer 104 can be directly formed ontop of the substrate 102 without the deposition of a nucleation layer103. The substrate temperatures during growth are typically 550 degreesC. for the GaN nucleation layer, 1000 degrees C. to 1100 degrees C. forthe GaN and AlGaN layers and 700 degrees C. to 800 degrees C. for theInGaN layers. In addition, the reactor pressure may be controlledbetween 50 Torr and 740 Torr. As organometallic precursors for the MOCVDgrowth TMGa (trimethylgallium), TMAl (trimethylalurninum), TMIn(trimethylindium) and TEGa (triethylgallium) are used for the group IIIelements and NH₃ (ammonia) is used as the nitrogen source. Hydrogenand/or nitrogen are used as carrier gas for the metalorganic sources.For the n-doping, 100 ppm SiH₄ diluted in H₂ is used, and for thep-doping, Cp₂Mg (cyclopentadienylmagnesium) is used. Other examples ofp-type dopants include, but are not limited to, Mg, Ca, C and Be.Examples of n-type dopants include, but are not limited to, Si, O, Se,Te and N-vacancies.

After MOCVD growth, the Mg p-doping in (Al)GaN:Mg layers is activated byRTA (rapid thermal annealing) at 850 degrees Celsius for 5 minutes in N₂ambient. A ridge waveguide structure is formed by dry-etching into thep-GaN waveguide layer 110 with CAIBE (chemical assisted ion beametching) or RIE (reactive ion beam etching) in anAr(argon)/Cl₂(chlorine)/BCl₃(borontrichloride) gas mixture as shown inFIG. 4a.

In order to form the gain and the modulator section of the device, anisolation trench 114 is etched into the ridge-waveguide as shown in FIG.4b. The isolation trench 114 can be formed in the same step as theridge-wave-guide etch or can be formed in a separate step in order toobtain a different etch depth. An electrically insulating layer (e.g.,silicon-oxy-nitride or SiO₂ or Si₃N₄) (not shown in the Figure) can bedeposited, e.g. by plasma-enhanced chemical vapor deposition (PE-CVD),into the trench in order to reduce the disturbance of the optical modeand serve as passivation layer for the exposed surfaces.

Dry-etching using CAIBE (chemical assisted ion beam etching) or RIE(reactive ion beam etching) in an Ar/Cl₂/BCl₃ gas mixture is used toaccess the GaN:Si layer 104 for n-contact formation. An n-contact 120 isformed over the second III-V nitride layer 104, which is functioning asa lateral contact layer.

The n-electrode 120 of FIG. 4a is common to both the amplifier region116 and the modulator region 118. The n-contact metal can be depositedby thermal evaporation, electron-beam evaporation or sputtering.Typically Ti/Al, Ti/Au or Ti/Al/Au are used as n-metal contacts. Then-contacts are annealed in N₂ ambient at 500 degrees Celsius in order toreduce the contact resistance. A dielectric isolation layer 113 (e.g.silicon-oxy-nitride or SiO₂ or Si₃N₄) is deposited by plasma-enhancedchemical vapor deposition (PE-CVD) on top of the ridge-waveguide.Alternatively polyimide can also used for isolation. Windows forp-contact formation are etched into the dielectric isolation layer usingRF plasma etching in a CF₄/O₂ ambient.

In FIGS. 4a and 4b, a p-electrode 200 is deposited on top of theamplifier contact layer 201 for the amplifier region 116. A p-electrode300 is deposited on top of the modulator contact layer 301 for themodulator region 118. The two p-electrodes 200 and 300 are separate anddistinct and allow for independent addressability of the amplifierregion 116 and the modulator region 118. Ni/Au, NiO/Au, Pd/Au,Pd/Au/Ti/Au, Pd/Ti/Au, Pd/Ni/Au, Pt/Au or Pd/Pt/Au can be deposited asp-contact metal by thermal evaporation, electron-beam evaporation orsputtering.

The laser facets can be formed either by cleaving or dry-etching (e.g.CAIBE). A SiO₂/TiO₂, SiO₂/Ta₂O₅ or SiO₂/HfO₂ high reflective coating canbe deposited on the backside of laser diode facets 126 by e-beamevaporation in order to enhance the mirror reflectivity. A SiO or SiO₂anti-reflective coating can be deposited on the front side of the laserdiode facet 124 using e-beam evaporation.

As seen in FIGS. 4b and 5, the trench 114 separates the laser structure100 into an amplifier region 116 and a modulator region 118. Theamplifier region 116 has a metal p-contact 200, a p-contact layer 201,an upper cladding layer 202, an upper waveguide layer 203 and an upperconfinement layer 204 The modulator region 118 has a metal p-contact300, a p-contact layer 301, an upper cladding layer 302, an upperwaveguide layer 303 and an upper confinement layer 304.

The amplifier p-contact layer 201 is separate and distinct from themodulator p-contact layer 301 but both are formed from the p-contactlayer 112 before the groove 114 is etched. The amplifier upper claddingand waveguide layers 202 and 203 are separate and distinct from themodulator upper cladding and waveguide layers 302 and 303 but both areformed from the upper confinement and waveguide layer 110 and 111 beforethe groove 114 is etched. The amplifier upper confinement layer 204 isseparate and distinct from the modulator upper confinement layer 304 butboth are formed from the p-upper confinement layer 109 before the groove114 is etched.

The active quantum well layer 108, the lower waveguide layer 107, thelower cladding layer 106, the defect reducing layer 105, the thick(Al)GaN current spreading layer 104, the buffer layer 103 and thesubstrate 102 are common to both the amplifier region 116 and themodulator region 118.

The optimum depth of the isolation trench 114 will depend on whethercomplete electric isolation is desired or minimum disturbance of theoptical mode. Complete electric isolation is obtained if the trench 114is etched through all the p-type layers and may even reach into orbeyond the MQW active region 108. This might, however, disturb theoptical mode traveling between the modulator and the gain section of thedevice and lead to scattering losses. If the trench is only partiallyetched into the p-type layers some electric connection between the gainand modulator section remains. The cross-talk between these two section,however, is expected to be quite small, because of the high lateralresistance of the p-type GaN and AlGaN layers. For example, if thetrench 114 is only etched into the p-AlGaN cladding layer 111 (as shownin FIG. 4b), the remaining GaN:Mg waveguide layer 110 and p-AlGaN layer109 would yield a series resistance of about 3 MΩ between the modulatorand the gain section (assuming a waveguide thickness of 100 nm, a trenchwidth of 10 μm and a ridge-waveguide width of 2 μm, a hole concentrationof 10¹⁸ cm⁻³ and a mobility of 1 cm²/Vs). The trench can also berefilled with an electrically insulating dielectric layer (e.g.silicon-oxy-nitride or Si₃N₄) (not shown in the Figure), which can bedeposited by plasma-enhanced chemical vapor deposition (PE-CVD)) inorder to improve the optical coupling between the modulator and the gainsection.

Alternatively, electric isolation between the modulator and gain sectioncan be obtained by ion implantation and without the etching of a trench.Ion implantation of the area 115 between the modulator and the gainsection for example with protons (H⁺) or oxygen ions would make thisarea electrically insulating and have the additional benefit that theoptical waveguide would not be disturbed. An example of such a structureis shown in FIG. 4c. The ion implanted region could reach into or beyondthe InGaN MQW active region in order to prevent and carriers from thegain section leaking into the modulator section of the device.

The light beam 128 emitted by the laser structure 100 is emitted fromthe mirror 124 adjacent to the modulator region 118 in order to minimizeany spontaneous emission from the amplifier region 116 in the OFF state.

For this invention, the active layer 108 under the amplifier region 116is the amplifier active layer 130 and the active layer 108 under themodulator region 118 is the modulator active layer 132.

In the two region blue laser structure 100, the amplifier region 116 isstrongly pumped with current to serve as a light emitting region, andthe modulator region 118 is pumped with a lower current level than theamplifier region to allow high frequency modulation. Alternatively themodulator region can be also reverse biased, which also enables highfrequency operation.

The amplifier region 116 is of much greater length than the modulatorregion 118. Accordingly, the active layer 130 of the amplifier region116 provides essentially all of the gain required to produce the desiredoutput intensity. The active layer 132 of the modulator region 118controls the output of laser 100 by switching the internal loss from ahigh value to a low value.

The amplifier region 116 will be forward biased by an input currentapplied through the p-electrode 200 and the n-electrode 120. The currentwill cause electrons to flow from the n-doped layers of the currentspreading layer 104, the defect reducing layer 105, the lower claddinglayer 106 and lower waveguide layer 107 into the amplifier active layer130. The current also causes holes to flow from the p-doped layers ofthe amplifier contact layer 201, the amplifier upper cladding layer 202,the amplifier upper waveguide layer 203 and the amplifier upper currentconfinement layer 204 into the amplifier active layer 130. Recombinationof the electrons and holes in the amplifier active layer 130 at asufficient current will cause stimulated emission of light 128.

The current applied to the amplifier region 116 is adjusted so thatenough gain is generated in the amplifier active layer 130 to overcomethe total optical loss including the mirror loss and the optical lossfrom of the entire active layer 108 in the two section laser structure100, when the modulator section 118 is in the ON state (ON state=themodulator active layer 132 is in the low-loss-state) but is notexceeding the total optical loss including the mirror loss and the lossof the entire active layer 108 if modulator region 118 is in the OFFstate (OFF state=the modulator active layer 132 is in thehigh-loss-state). The amplifier current is kept constant. Thetwo-section laser structure 100 modulates the laser emission and laseroutput power by varying the optical loss in the active layer 132 of themodulator section of the laser device. If the modulator section is itshigh-loss-state (OFF state), the gain produced in the amplifier sectionis not large enough to overcome the total optical loss and thereforelasing is prohibited. If the modulator is switched to its low-loss-state(ON state) the optical gain produced in the amplifier section will belarge enough to overcome the total optical loss and lasing will beallowed. The optical loss in the modulator section can be either variedby applying a forward current or by applying a reverse bias voltage tothe modulator region 118.

There are two basic modes of modulation for the two-section blue laserstructure 100 of the present invention.

One mode, called the “forward current modulation mode”, is one in whichthe amplifier region 116 is sufficiently forward biased to causestimulated light emission and a negligible minimal forward bias currentis applied to the modulator region. The modulator region 118 can beforward biased by an input current applied through the p-electrode 300and the n-electrode 120. The current will cause electrons to flow fromthe n-doped layers of the current spreading layer 104, the defectreducing layer 105, the lower cladding layer 106 and lower waveguidelayer 107 into the modulator active layer 132. The current also causesholes to flow from the p-doped layers of the modulator contact layer301, the modulator upper cladding layer 302 and the modulator upperwaveguide layer 303, and the modulator current confinement layer 304into the modulator active layer 132. If no current is applied, themodulator section 118 is in the high-loss-state and lasing is prohibited(OFF state). Injection of the electrons and holes in the modulatoractive layer 132 at a sufficient current will reduce the loss in themodulator active layer 132. As the modulator region 118 is in itslow-loss-state, the gain from the amplifier section 116 will besufficiently high to overcome the total loss and lasing is permitted (ONState).

Another mode, called the “reverse bias modulation mode”, also has theamplifier region sufficiently forward biased to cause stimulatedemission, but has a reverse bias voltage applied to the modulatorregion.

The reverse bias input voltage to the modulator region 118 can beapplied through the p-electrode 300 and the n-electrode 120. By applyinga reverse bias to the modulator region p-n junction formed by the lowerwaveguide layer 107, the InGaN MQW active region 1132 and the uppercurrent confinement layer 304, the electric field in the modulatorsection p-n junction can be changed. If no voltage is applied, themodulator section 118 is in the high-loss-state and lasing is prohibited(OFF state). As the modulator section 118 is in its low-loss-state, byapplying an external reverse bias, the gain from the amplifier section116 will be sufficiently high to overcome the total loss and lasing ispermitted (ON state).

Examples of the measured laser diode characteristics of devicesoperating in the “reverse bias modulation mode” are shown in FIGS. 6 and7. The amplifier region in this example has a length of 700 μm, themodulator region has a length of 100 μm and the ridge-waveguide has awidth of 3 μm. Amplifier and modulator region are separated by a 20 μmwide trench, which was etched into the upper GaN waveguide layer 110.FIG. 6a shows the light output vs. current characteristic for such atwo-section laser device with different reverse bias voltages applied tothe modulator section. With no voltage applied to the modulator section,the loss in the modulator section is high resulting in a high thresholdcurrent (I_(th)˜225 mA). If a reverse bias voltage is applied to themodulator section the loss in the modulator section is reduced leadingto a reduction of the threshold current. For example at a modulatorvoltage of U_(mod)=6 V the threshold current is reduced to I_(th)˜190mA). In the “reverse bias modulation mode”, the laser is operated withthe current in the gain (amplifier) section set to a constant current ofI_(gain)=225 mA (indicated by the dotted curve in FIG. 6a). The lightoutput vs. modulator voltage characteristic of the same two-sectionlaser diode is shown in FIG. 6b. The current in the gain (amplifier)section was set to a constant current value of I_(gain)=225 mA. As thereverse bias voltage applied to the modulator section increases, theloss in the modulator section is reduced and the light output increasesfrom 0.5 mW (at zero bias) to ˜3 mW (at U_(mod)=7.2 V). Thecorresponding laser diode emission spectra are shown in FIGS. 7a and 7b.At a modulator section voltage U_(mod)=0 V the loss in the modulatorsection is high, thus prohibiting lasing (as shown in FIG. 7a). If themodulator section reverse bias U_(mod) is raised to 6 V, the loss themodulator section is reduced and consequently lasing is permitted, ascan be seen in the spectra of FIG. 7b.

The varying modulator region current and the varying dissipated electricpower in the modulator region is significantly smaller in comparison tothe constant current and the dissipated electric power in the amplifierregion. For example, if the two-section laser diode is operated in the“forward current modulation mode”, the current density necessary tomodulate the absorption in the modulator section (typically 100 A/cm² to(1000 A/cm²) is only a fraction of the current density necessary toproduce sufficient gain in the amplifier section (typically 2000 A/cm²to 5000 A/cm²). Furthermore the length of the modulator section is muchsmaller (typically 1/10 to ⅕ of the length of the amplifier section) andtherefore the current to switch the modulator section is even smaller.In the case when the two-section laser diode is operated in the “reversebias modulation mode”, the dissipated electric power in the modulatorregion will be even smaller. Since the modulator section is operated inreverse bias, no current is injected in the modulator section activeregion. The only current flowing in the modulator section is thephotocurrent, which is induced by the absorbed light from the amplifierregion. Accordingly, the laser structure 100 operates at an elevated butconstant temperature due to the constant amplifier region current. Thevarying modulator region current will have minimal temperature effect tothe laser structure 100. Modulating the laser from the non-lightemitting, or OFF state, to the light emitting, or ON state, does -onlyresult in a small increase in the operating temperature of the laser.

Since only the smaller modulator region 118 is used to control theoutput power of the emitted laser beam, the resulting lower capacitancewill help achieve higher modulation speeds for the blue laser diode 100.

The laser diode structure according to the invention described above canbe applied to any device requiring compact laser structures, includinghigh resolution laser printing devices, digital printers, displaydevices, projection displays, high density optical storage devices,including magneto-optical storage devices, including CD-ROM and DVD'swhereby data is stored on a magneto-optical disk, fiber-opticcommunications devices, including for fiber optic emitters and repeatersand undersea communications devices (sea water is most transparent inthe blue-green spectrum). The LED structure according to the inventioncan also be applied to any device requiring compact LED structures,including illumination devices and full color displays, includingmonolithically integrated pixels for full color displays.

While the invention has been described in conjunction with specificembodiments, it is evident to those skilled in the art that manyalternatives, modifications and variations will be apparent in light ofthe foregoing description. Accordingly, the invention is intended toembrace all such alternatives, modifications and variations as fallwithin the spirit and scope of the appended claims.

1. A semiconductor laser structure comprising: a sapphire substrate; aplurality of III-V nitride semiconductor layers formed on said sapphiresubstrate such that at least one of said plurality of III-V nitridesemiconductor layers forms an active layer; a first active region formedin said active layer, including at least one junction between p-type andn-type material, for functioning as an amplifier; a second active regionformed in said active layer, including at least one junction betweenp-type and n-type material, for functioning as an optical modulator;wherein a sufficient forward bias is applied to said first active regionsuch that stimulated emission is caused to occur therein, a portion ofsaid stimulated emission being directed into said second active region,and the optical loss of said second active region is varied to causelasing from said semiconductor laser structure.
 2. The semiconductorlaser structure of claim 1 wherein said optical loss of said secondregion of said semiconductor laser structure is varied by applying aforward bias current to said second active region.
 3. The semiconductorlaser structure of claim 1 wherein said optical loss of said secondregion of said semiconductor laser structure is varied by applying areverse bias voltage to said second active region.
 4. The semiconductorlaser structure of claim 1 further comprising a trench formed in atleast one of said plurality of III-V nitride semiconductor layers, saidtrench forming said first active region and said second active region.5. The semiconductor laser structure of claim 2 further comprising atrench formed in at least one of said plurality of III-V nitridesemiconductor layers, said trench forming said first active region andsaid second active region.
 6. The semiconductor laser structure of claim3 further comprising a trench formed in at least one of said pluralityof III-V nitride semiconductor layers, said trench forming said firstactive region and said second active region.
 7. The semiconductor laserstructure of claim 4 wherein said trench is formed in said plurality ofIII-V nitride semiconductor layers to said active layer.
 8. Thesemiconductor laser structure of claim 1 wherein said semiconductorlaser structure emits light with a wavelength within a range including360 nm to 650 nm.
 9. A semiconductor laser structure comprising: asubstrate; a plurality of III-V nitride semiconductor layers formed onsaid substrate such that at least one of said plurality of III-V nitridesemiconductor layers forms an active layer; a first active region formedin said active layer, including at least one junction between p-type andn-type material, for functioning as an amplifier; a second active regionformed in said active layer, including at least one junction betweenp-type and n-type material, for functioning as an optical modulator;wherein a sufficient forward bias is applied to said first active regionsuch that stimulated emission is caused to occur therein, a portion ofsaid stimulated emission being directed into said second active region,and the optical loss of said second active region is varied to causelasing from said semiconductor structure.
 10. The semiconductor laserstructure of claim 9 wherein said optical loss of said second region ofsaid semiconductor laser structure is varied by applying a forward biascurrent to said second active region.
 11. The semiconductor laserstructure of claim 9 wherein said optical loss of said second region ofsaid semiconductor laser structure is varied by applying a reverse biasvoltage to said second active region.
 12. The semiconductor laserstructure of claim 9 further comprising: a trench formed in at least oneof said plurality of III-V nitride semiconductor layers, said trenchforming said first active region and said second active region.
 13. Thesemiconductor laser structure of claim 10 further comprising: a trenchformed in at least one of said plurality of III-V nitride semiconductorlayers, said trench forming said first active region and said secondactive region.
 14. The semiconductor laser structure of claim 11 furthercomprising a trench formed in at least one of said plurality of III-Vnitride semiconductor layers, said trench forming said first activeregion and said second active region.
 15. The semiconductor laserstructure of claim 12 wherein said trench is formed in said plurality ofIII-V nitride semiconductor layers to said active layer.
 16. Thesemiconductor laser structure of claim 9 wherein said semiconductorlaser structure emits light with a wavelength within a range including360 nm to 650 nm.